This invention relates to climate control in a motor vehicle, and more particularly to a method of operation for automatically preventing windglass fogging.
In general, vehicle climate control systems include a controller that regulates a number of parameters such as blower motor speed, refrigerant compressor activation and/or capacity, air mixing door position, and discharge temperature. In a manual system, the operator directly or indirectly controls the parameters, while in an automatic system, the parameters are automatically controlled in response to a number of inputs, including cabin air temperature, outside air temperature and solar loading, to regulate the cabin air temperature at a set temperature selected by the operator. In either type of system, front and rear windglass defogging functions are ordinarily manually activated by the operator when the perceived desirability of defogging becomes apparent.
The desirability of providing automatic activation of front and rear defogging functions has been recognized in the prior art. See, for example, the U.S. Patents to Adiparvar et al. U.S. Pat. No. 5,653,904 and Tsunokawa et al. U.S. Pat. No. 5,701,752. Adiparvar et al. utilize front glass and rear glass moisture detection sensors in an otherwise conventional system for respectively activating front and rear defogging, while Tsunokawa et al. disclose a heat pump system in which windglass defogging is automatically activated when the measured windglass temperature is lower than the dew point.
While the above-described controls can be used to automatically trigger windglass defogging, they produce abrupt transitions in control functionality, and completely override other desirable control functions such as cabin temperature control, energy efficiency, and air quality management. Accordingly, what is needed is a method of automatically adjusting the operation of a climate control setting to prevent windglass fogging, without producing unnecessarily abrupt or large deviations from the climate control setting otherwise in effect.
The present invention is directed to an improved vehicle climate control system that develops a fog factor signal indicative of the relative potential of windglass fogging, and uses the index to schedule offsets to the normal control settings. According to the invention, the air dewpoint temperature at or near the front windglass is estimated based on relative humidity and a reference temperature, and the fog factor is determined as a function of the estimated air dewpoint temperature and a measure of the windglass temperature. In the illustrated embodiment, the fog factor is used to offset an air inlet door, mode doors, refrigerant compressor capacity, discharge air temperature, and blower motor speed, as well as to activate any defrost functions on the side glass or rear glass. Advantageously, the control of this invention has priority over existing control settings, regardless of the specific climate control, energy efficiency, or air quality algorithms that may be in effect.